This report looks at the problems surrounding sanitation in our rapidly urbanising world and highlights the health threats caused by 700 million urban dwellers worldwide living without a toilet. Human beings are now largely an urban species: for the first time in history, more than half of the world's population (54%, or 3.9 billion people) lives in towns cities and megacities. By 2050, that's expected to rise to two thirds. Many new urbanites, and particularly the poorest are not moving into gleaming apartment blocks or regenerated post-industrial areas. They are arriving – or being born into – overcrowded, rapidly expanding slums. Economic growth is usually driven by urbanisation, and all industrialised countries already have a mostly urban population. This means that nearly all the current urban population is happening in developing countries. UN Habitat estimates that more than one-third of the developing world's urban population – over 863 million people – live in slums. Often, city planning and infrastructure building have been unable to keep pace.